A. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and a semiconductor device.
B. Description of the Related Art
Semiconductor devices with a silicon on nothing (SON) structure have been disclosed in Japanese Unexamined Patent Applications Publication Nos. 2001-144276, 2003-095797, and 2004-111766, for example. The SON structure includes a cavity (cavities) within a semiconductor substrate of silicon, silicon carbide, gallium nitride, or other semiconductor material. Use of surface migration of silicon atoms has been proposed for manufacturing a semiconductor device with the SON structure. The method conducts surface diffusion of silicon atoms in the surface region of the semiconductor substrate by heat treating the substrate in a hydrogen atmosphere or in a high vacuum environment. The heat treatment eliminates defects on the surface region of the semiconductor substrate on an atomic level and flattens the surface.
The following describes a method of forming an SON structure by utilizing the surface migration of silicon atoms with reference to FIGS. 10A, 10B, and 10C, which illustrate sequentially a conventional method of manufacturing a semiconductor device with an SON structure in the order of the manufacturing processes. FIG. 10A is a plan view showing a state of the manufacturing processes; FIG. 10B is a sectional view cut along the line AA-AA′ in FIG. 10A; and FIG. 10C is a sectional view showing the SON structure formed by a process subsequent to the process shown in FIG. 10A and FIG. 10B.
First, the oxide film formed on the front surface of semiconductor substrate 101 by thermal oxidation is patterned to form a mask oxide film (not shown in the figure) for trench etching process. Then, an anisotropic etching process is conducted using the mask oxide film in the surface region of semiconductor substrate 101 to form a plurality of trenches 102 in a planar layout of a matrix with a depth that does not reach the back surface of semiconductor substrate 101.
After that, a heat treatment process is conducted on the front surface of semiconductor substrate 101 in a hydrogen atmosphere at a temperature of 1,150° C. generating surface migration of silicon atoms in the front surface region of semiconductor substrate 101. Resulting from the surface migration process, the top parts opposite to the bottom parts of trenches 102 are closed to form a plurality of small cavities (not shown in the figure) in semiconductor substrate 101. Further heat treatment joins these plural small cavities into one cavity 103 having a shape of a flat plate with a longitudinal direction parallel to the principal surface of semiconductor substrate 101. Thus, an SON structure is constructed.
Japanese Unexamined Patent Application Publication No. 2001-144276 discloses a method for forming an SON structure utilizing the surface migration of silicon atoms. The method comprises a step of forming a plurality of first trenches in the surface region of a semiconductor substrate, and a step of heat treating the substrate to transform the plurality of first trenches into a single cavity having a shape of a flat plate. The aspect ratio of the first trenches needs to be at least 5. For forming a cavity with a shape of a tube or a form of plate, the first trenches need to be arranged in the shape of a line or a lattice. The distance D between adjacent two first trenches needs to be set below 4R, where R is the radius of the first trench.
Japanese Unexamined Patent Application Publication No. 2006-075981 discloses a device manufactured by another method in which a cavity is formed through at least one access hole during a second period of a trench etching process. At that time, a second trench step is conducted over a set second duration during the second period, and the second duration is set longer than a first duration.
In conventional methods for forming the SON structure described above, however, the thickness of cavity 103 is restricted by the depth of trench 102 and the planar layout of trench 102. Japanese Unexamined Patent Application Publication No. 2001-144276, for example, proposes to arrange the trenches in a planar layout of a stripe shape or a matrix shape with a dimensional conditions of an aspect ratio of at least 5 and a dimensional relationship of D<4R, where D is a distance between adjacent trenches and R is a radius of the trench, in order to transform the trenches into a cavity in a shape of a tube or a plate.
In addition, FIG. 17 of Japanese Unexamined Patent Application Publication No. 2001-144276 shows that the number of cavities transformed from the trenches depends on the depth of the trenches and the distance between the adjacent trenches, and that two cavities are formed from the trenches with an aspect ratio of around 8. Although the SON structure formed by a conventional method forms a relatively thick cavity from trenches of relatively high aspect ratio, the trenches formed with a high aspect ratio result in a plurality of separate cavities. Thus, an SON structure including a thick single cavity can hardly be formed by the conventional methods. Moreover, the conventional methods of forming an SON structure have failed to form a cavity in a flat plate shape with a thickness more than 3 μm. For example, if a diaphragm for pressure detection is manufactured using a semiconductor substrate having a cavity and the cavity does not have a necessary thickness, the diaphragm comes into contact with the bottom of the cavity when the diaphragm is subjected to a pressure and the silicon substrate is deformed, making the pressure measurement impossible.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.